Molecules, Compounds, and Chemical Reactions

I. Molecules and Compounds

A compound is a substance consisting of two or more different atoms, in specific proportions, bonded together in a specific pattern.

A molecule is the smallest part of a compound that retains all of the properties of that compound.

Molecular formulas are a shorthand way to represent the types and numbers of different atoms present in a molecule. Examples are H₂O, water; CO₂, carbon dioxide; C₆H₁₂O₆, glucose, fructose, galactose.

Molecular formulas provide information on the proportion of atoms in a molecule, but not the pattern of bonding. While this does not cause a problem with small molecules (H₂O or CO₂) it can with larger molecules such as C₆H₁₂O₆. This molecular formula is the same for the sugars glucose, galactose and fructose. Compounds with the same molecular formula, but different structures are called isomers. The figure below shows the isomers of the hexose sugars.

II. Molecular Weight and Avagadro's Number

The molecular weight of a molecule is equal to the sum of the atomic masses of all the atoms in a molecule

water H₂O (1x2) + 16 =18 daltons

carbon dioxide CO₂ (16x2) + 12 = 44 daltons

sucrose C₁₂H₂₂O₁₁ (12x12) + (1x22) + (16x11) = 342 daltons

insulin C₂₅₄H₃₇₇N₆₅O₇₅S₆ = 5727 daltons

Numbers of Molecules and Concentration

In most biological systems what we are usually concerned with is the number of molecules of a substance present, not the weight. To measure the concentration biologists use the concept of Gram Molecular Weight (Mole). The amount of a compound equal to its molecular weight in grams always contains the same number of molecules (6.023 x 10²³, Avogadro's Number) 1 mole (symbolized by M).

18 grams of water contains the same number of molecules as

44 grams carbon dioxide

342 grams sucrose

5727 grams insulin

All of these quantities contain 1 mole of molecules

In solution the concentration of a compound is measured by the number moles/liter of solution (molarity). To prepare a 1 molar sucrose solution you would combine 342 grams of sucrose with enough water to make a final volume of 1 liter. It is important to remember that you do not add 1 liter of water to the sucrose, but rather enough water to make a final volume of 1 liter.

Try these examples.

II. Chemical Reaction

During chemical reactions one or more molecues may be converted into different molecule/s, but the total quantity of matter must remain the same. In the chemical reaction below note that there are the same numbers of each atom on both sides of the equation. This is true for all chemcial reactions.

Also note that in this reaction there are arrows pointing in both directions. Like most chemical reactions this one is reversible.

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